Breast cancer screening programmes may have no effect on death rates, researchers said today.
Danish experts cast doubt on the benefits of mammography after research showed few differences between women who were screened and those who were not screened when it comes to deaths from breast cancer.
It follows research last July by the same team, which concluded that one in three breast cancers detected by screening may actually be harmless.
That study, based on data from Britain, Canada, Australia, Sweden and Norway, suggested some women undergo unnecessary treatment for cancers that are unlikely to kill them or spread.
Some cancers grow so slowly that the patient dies of other causes first, or the cancer remains dormant or regresses.
The researchers, from the Nordic Cochrane Centre in Denmark, said cancer screening programmes could lead to "overdiagnosis".
In their latest 10-year study, published online in the British Medical Journal (BMJ), experts from the Centre found death rates from breast cancer fell 1pc per year in screened areas among Danish women aged 55 to 74, and by 2pc in non-screened areas.
In women too young to benefit from screening (aged 35 to 54), breast cancer death rates fell by 5pc per year in the screened areas and by 6pc per year in the non-screened areas.
For older age groups (75 to 84), there was little change over time both in screened and non-screened areas.
The authors wrote: "The reductions in breast cancer mortality we observed in screening regions were similar or less than those in non-screened areas and in younger age groups, and are more likely explained by changes in risk factors and improved treatment than by screening mammography."
The researchers said their findings were similar to results from other countries.
"In the UK, where screening started in 1988, the decline in breast cancer mortality from 1989 until 2007 was 41pc in women aged 40-49 years, who were not invited to screening, 41pc in women aged 50-64 years, who were invited to screening from 1988, and 38pc in women aged 65-69 years, who were invited from 2002.
"Furthermore, the drop in breast cancer mortality in the relevant age group began before the screening programme started."